As the semiconductor industry continually strives to increase the power density of single chip packages, thermal management remains a critical challenge toward realizing both performance and reliability metrics. One of the problems inhibiting effective thermal management is the several interfaces that can exist between the chip and heat sink. Specifically, the thermal resistance of the thermal interface materials (TIMs) that are currently used to bridge these interfaces must be decreased.
Carbon nanotubes (CNTs) with their extraordinarily high axial thermal conductivity have generated tremendous interest as candidates for low resistance TIMs. The most promising CNT TIMs produced to date contain vertical forests, where the CNT axis is nominally aligned orthogonal to the contact surfaces providing maximum conductivity in this direction. This alignment also provides maximum mechanical compliance along the contact surfaces to mitigate deleterious effects of mismatches in the coefficients of thermal expansion of the interface materials. However, even in this arrangement CNT TIMs demonstrated limited performance due to the presence of high thermal contact resistances between the CNT tips and opposing surfaces.
Efforts to mitigate this contact resistance have centered on different methods for bonding the CNT tips to the opposing surface and have included metallic film bonding, palladium nanoparticle bonding, and wet chemical modification of Si. While these techniques have in some cases produced CNT TIMs with thermal resistances approaching those of conventional TIMs, they often require high-cost materials, such as Au or In, and processing, including metal thin film deposition, high temperature and high pressure bonding, and even exposure to microwave radiation, that aren't favorable for large-scale implementation. Moreover, these processes typically result in a permanent modification to the CNT tips which cannot be reworked or removed.
There exists a need for a scalable, low-cost process for improving the thermal resistance of carbon nanotubes and devices made by these methods.
Therefore, it is an object of the invention to provide a scalable, low-cost process for improving the thermal resistance of carbon nanotubes and devices formed by the method.
It is also an object of the invention to provide methods for reducing the thermal resistance of CNT arrays or sheets, wherein the methods result in a coating or modification which can be removed to form a clean surface which can again be coated or modified.